1. Field of the Invention
The present invention relates generally to a positive electrode plate for a nickel-hydrogen cell wherein a foaming nickel substrate is filled with a paste-like kneaded substance containing nickel hydroxide as a principal component. More specifically, the invention relates to an improvement in the reduction of the ohmic resistance of the positive electrode plate.
2. Description of the Related Art
FIG. 7 shows the internal structure of an enclosed type nickel metal hydride (Ni MH) cell as a typical nickel-hydrogen cell, to which the present invention is applied. The enclosed type nickel metal hydride cell shown in FIG. 7 has a cylindrical container 4. This container 4 houses therein a positive electrode plate 5, a negative electrode plate 6, and a separator 7, which are cylindrically wound. In addition, the container 4 is filled with a predetermined electrolyte.
The positive electrode plate 5 is formed of a plate material which is formed by filling a foaming nickel substrate 1 with a paste-like kneaded substance containing nickel hydroxide as a principal component and pressure forming the resulting substrate after drying. This plate material is cut so as to have a predetermined dimension, and the cut material (together with the negative electrode 6 and the separator 7) is cylindrically wound to form the positive electrode plate 5.
The foaming nickel substrate 1 constituting the positive electrode plate 5 is prepared by removing a plate foaming core material 2 of urethane foam or the like after nickel-plating the foaming core material 2 while tension is applied to the foaming core material 2 in specific direction.
From the point of view of an improvement in the energy density of the nickel-hydrogen cell, it is required to reduce the weight ratio (increase the porosity rate) of the foaming nickel substrate 1 to the positive electrode plate. For that reason, the mass per unit area of nickel in the foaming nickel substrate 1 of the conventional positive electrode plate 5 is limited to about 0.35 to 0.5 kg/m2.
However, in the above described conventional positive electrode plate for a nickel-hydrogen cell, there is a problem in that the ohmic resistance of the positive electrode plate is too great from the point of view of an improvement in discharge characteristics, although it is suited to improve the energy density of the nickel-hydrogen cell.
In addition, recent positive electrode plates themselves are harder with the densification of positive electrode plates, so that winding cracks are easily caused in the positive electrode plate when it is cylindrically wound. Then, the winding cracks in the positive electrode plate progress due to the expansion and contraction of the positive electrode plate caused by subsequent charge and discharge cycles, so that the ohmic resistance of the positive electrode plate is increased to deteriorate the discharge characteristics of the nickel-hydrogen cell.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a positive electrode plate capable of having a lower ohmic resistance than those of conventional positive electrode plates, so as to reduce the internal resistance of a nickel-hydrogen cell using the positive electrode to improve the discharge characteristics thereof.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a positive electrode plate for a nickel-hydrogen cell, the positive electrode plate comprising: a foaming nickel substrate formed by removing a plate foaming core material after nickel-plating the foaming core material; and a kneaded substance containing nickel hydroxide as a principal component, the kneaded substance being filled in the foaming nickel substrate, wherein the foaming nickel substrate having a mass per unit area of nickel of 0.5 to 1.1 kg/m2.
According to this positive electrode plate, the ohmic resistance of the foaming nickel substrate can be lower than those of conventional foaming nickel substrates by setting the mass per unit area of nickel in the foaming nickel substrate to be 0.5 kg/m2 or higher.
On the other hand, if the mass per unit area of nickel in the foaming nickel substrate is too great, the energy density of the nickel-hydrogen cell is remarkably decreased due to the increase of the weight ratio of the foaming nickel substrate to the positive electrode plate. In addition, if the mass per unit area of nickel exceeds a certain extent, the reduction of the ohmic resistance of the foaming nickel substrate is not so great.
Therefore, in the positive electrode plate according this aspect of the present invention, the upper limit of the mass per unit area of nickel in the foaming nickel substrate is set to be 1.1 kg/m2 to prevent the reduction of the energy density from exceeding the effect of the reduction of the ohmic resistance of the foaming nickel substrate.
According to another aspect of the present invention, there is provided a positive electrode plate for a nickel-hydrogen cell, the positive electrode plate being formed of a plate material comprising: a foaming nickel substrate formed by removing a plate foaming core material after nickel-plating the foaming core material while tension is applied to the foaming core material in specific direction; and a kneaded substance containing nickel hydroxide as a principal component, the kneaded substance being filled in the foaming nickel substrate, wherein the plate material is cut so as to have a predetermined dimension, and cylindrically wound in a winding direction which is substantially perpendicular to the direction of tension applied to the foaming core material.
According to this positive electrode plate, the elongation percentage of the foaming nickel substrate in direction substantially perpendicular to tensile direction applied to the foaming core material is greater than that in the tensile direction. Therefore, by causing the former direction to be coincident with the winding direction of the positive electrode plate, the flexibility of the positive electrode plate with respect to the winding can be maximum to prevent winding cracks from being caused in the positive electrode plate.